No, the 598 isn't necessarily a better listening experience than the 600, you're right.
I'd say overall, the value proposition of the 600 is better than that of the 598, at least to my ears. Price wise, I think both phones fall correctly in Sennheiser's lineup.
They are both very good, and it's really critical that you get a good match with whatever phones, amp, and source.
They both can sound great, but only if you get good synergy in your system. Otherwise, they can tend to gravitate to "very good, but nothing special."
I do enjoy the 600, just that I like the HE-500 more. 598 is sufficiently different that I use those every now and then, but usually in another semi-portable setup.